Vol. 4 No. 2 Spring 2008 Journal of Double Star Observations Page 59 Visual Double Star Measurements with an Alt-Azimuth Telescope Thomas G. Frey California Polytechnic State University San Luis Obispo, CA 93407 Abstract: An alt-az mounted Newtonian telescope was used to determine the separation and position angle of seven known and five neglected double stars. The problem of field rotation was solved by modifying the usual observing technique. Separation and position angle determinations are described, and the standard deviations and mean errors for these measurements are presented. The direction of future studies is outlined. techniques were accurate and precise, additional Introduction measurements on neglected double stars listed in the Professional astronomers have carried out visual Washington Double Star Catalogue were made. double star measurements for over 200 years. These scientists measured the separation between double Double Star Observation: Equatorial vs. stars in arc seconds, and the position angle in degrees Alt-Az Mounts that defined the orientation of pairs with respect to Most observers involved in double star measure- celestial north. Over time, the orbital motion of each ments, including Argyle (p.x) and Teague (p.112), star can create a change in the observed separation recommend the use of equatorial mounted telescopes. and position angle if the pair proves to be binary in Such telescopes have drive motors that are oriented so nature. A binary star revolves around a common the right ascension axis rotates around the north center of mass. celestial pole, canceling out the Earth’s rotation and Today’s amateur astronomers continue to evaluate the image in the eyepiece remains stationary. these changes with fairly simple equipment. The The equatorial telescope can be equipped with an usual recommended setup includes an equatorial illuminated reticle eyepiece such as the 12.5 mm mounted telescope with tracking motors, a laser- Celestron Micro Guide or the 12 mm Meade astromet- etched astrometric eyepiece, and a stopwatch. ric eyepiece. Both eyepieces have similar configura- This study, however, utilizes an altitude-azimuth tions: a linear scale in the middle and a 360° protrac- (alt-az) mounted telescope instead of an equatorial tor scale around the circumference of the field of the mounted telescope. Alt-az mounts are seldom used in eyepiece. The linear scales are divided into 60 and 50 double star measurements due to the rotation ob- equal divisions on the Celestron and Meade eyepieces, served in the field of view. This motion can affect both respectively. Sometimes an external protractor scale the accuracy and precision of the measurement of the is mounted to the base of the eyepiece to more accu- position angle. Yet, with minor adjustments made at rately measure the position angles, as described by regular intervals during the observing session, both Tanguay (p.116) and Johnson and Genet (p.147). separation and position angle measurements made on Separation between double stars is determined by known double stars correlate closely to literature using the slow motion control of the equatorial tele- values. Once it was determined that the measuring scope to align the pair on the linear scale. Then esti- Vol. 4 No. 2 Spring 2008 Journal of Double Star Observations Page 60 Visual Double Star Measurements with an Alt-Azimuth Telescope mate the number of divisions on the scale between the larCAT. This system is controlled with a Wildcat Argo centers of the stars to the nearest tenth. The number Navis computer. The Meade 12.5 mm astrometric of arc seconds represented by each scale division is eyepiece was used for all double star measurements. A previously determined by using either the drift RadioShack LCD Stopwatch with 0.01 second resolu- method with a star of known celestial coordinates, or tion was used to calibrate the linear scale of the calibration double stars alone, that have had no Meade eyepiece. change in separation in 50 years. The observation sessions were held in San Luis Position angles are measured again by using a Obispo, CA and Atascadero, CA. San Luis Obispo is slow motion control, this time to move the primary located at latitude 35°16’ N, altitude 360 feet, in an star to the center of the linear scale. The eyepiece is area where light pollution is extensive and an evening rotated until the secondary star is also on the linear marine layer limits the time of effective seeing. Atas- scale. The drive motors are turned off and the pair cadero is located approximately 15 miles north of San drifts across the field of view until the primary Luis Obispo at 35°30’N, altitude 1050 feet, in an area reaches the protractor scale at the outer edge of the of limited light pollution and no marine layer. field. The drive motors are re-engaged and the posi- tion angle is indicated by the position of the primary Calibration of the Meade Astrometric star on the protractor scale. Eyepiece The axis of a telescope attached to an alt-az mount The linear scale of the reticle eyepiece must be rotates about the zenith, not the celestial pole like an calibrated to the telescope being used. To determine equatorial mount. Even if the alt-az telescope is the number of arc seconds per division, Argyle (p. 152) equipped with drive motors, stars in the field of view recommends using a star of medium brightness rotate around objects in the center of the field. The (magnitude 5-6) with a declination of 60-75° and closer a celestial object is to the zenith, the faster the allowing it to pass along the length of the linear scale. rotation in the field. This rotation makes any type of This is carefully timed to the nearest 0.01 seconds. To imaging (astrophotography or CCD imaging) problem- reduce random errors in the process, 8-10 different atic. It also can affect double star measurements, drift times were recorded and the average determined. especially position angle determination. Field rotation The average drift time is used in the following can be compensated for by using a de-rotator attached equation: to the eyepiece focuser, but this requires an additional motor and adds additional complexity and expense to 15.0411T cos(δ ) Z = avg RS the instrumentation. D Argyle (p. 286) and Napier-Munn (p. 22) both point out that the rate of field rotation observed in alt- where Z is the scale constant in arc seconds per az mounted telescopes is greatest at the zenith and division, Tavg is the average drift time of the reference zero when a star crosses the prime vertical, that is, star across the scale in seconds, 15.0411 is the side- when the star is due east or due west. As a result, real motion in arc seconds per second of Earth’s rota- initial double star measurements were conducted in tion, cos(δRS) is the cosine of the declination of the an easterly direction and between altitudes of about reference star, and D is 50, the number of divisions for 20-80°. the Meade eyepiece It will be shown in this study that if simple ad- Both Alpha Cephei (Alderamin) and Gamma justments are made to the reticle eyepiece of an alt-az Cassiopeiae (Navi) were used as calibration stars, telescope during double star data collecting, excellent depending on the time of the year the observations results can be achieved in separation and position were conducted. Typical examples of the calibrations angle measurements that closely agree with data are indicted in Table 1. The units for the scale con- obtained with traditional equatorial telescopes. stant are arc seconds per division (a.s./div). Equipment Used Procedure for Measuring Separation The telescope used in this research was an Obses- After determining the scale constant for the sion f/4.5 18-inch Newtonian telescope with a Dob- Meade astrometric eyepiece, the telescope was two- sonian (or alt-az) mount. It was equipped with a star aligned and the tracking motors engaged. The ServoCAT tracking and GOTO system made by Stel- primary star was centered in the eyepiece and the Vol. 4 No. 2 Spring 2008 Journal of Double Star Observations Page 61 Visual Double Star Measurements with an Alt-Azimuth Telescope Bess. Dec. Av Drift Scale Constant Star # Obs. Std. Dev. Mean Error Epoch (°) Time(sec) (a.s/div) α Cephei B2007.618 62.58 16 88.17 0.283 0.071 12.21 γ Cass B2007.769 63.67 12 83.49 0.280 0.081 12.22 Table 1: Determination of the scale constant eyepiece rotated so that both stars were co-aligned on Final Procedure Adopted the linear scale. The number of divisions between the Instead of doing repetitive two-star alignments stars was noted and estimated to the nearest 0.1 with the Argo Navis computer after every position division. The slow motion control on the ServoCAT angle measurement, the tracking motors were disen- was then used to move the double star along the scale gaged after the separation measurements were con- to a new location and another reading recorded. This ducted. This allowed manual movement of the tele- was done to reduce the random error in assigning the scope. The eyepiece was rotated until the double stars number of divisions. Usually, no fewer than 10 read- were aligned on the linear scale. Then the telescope ings were recorded and the average number of divi- was moved so that the primary star accurately drifted sions calculated. The separation in arc seconds was through the central division mark. In practice, the calculated by multiplying the scale constant, Z, by the primary was situated about 5-8 division marks away average number of divisions between the double stars.